Method for Producing Metal Products

ABSTRACT

To produce products from metal, such as powders, foils, coatings and molded parts, such as pins, pipes or sheets, from metal in the form of a semifinished product ( 15 ), the metal of the semifinished product ( 15 ) is melted by an inductive magnetic field ( 12 ), atomized and allowed to solidify in a chamber ( 25 ) into a powder or sprayed onto a carrier and hardened on the carrier. The molten metal is supplied in a gas nozzle ( 10 ) which is made either as a Laval nozzle or as a Venturi nozzle, as a film ( 21 ) which is stabilized by gas flows ( 14 ), and is then atomized by other gas flows ( 13 ).

The invention relates to a process for producing products from metal, especially powders, foils, coatings and molded parts, such as pins, pipes or sheets, from metals, which are used in the form of a semifinished product.

The material technical properties of reactive metals such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhenium and their alloys and of superalloys (alloys based on nickel or cobalt) are largely determined by their purity, especially by the absence of oxides and ceramic impurities. Due to the high melting points of these metals and alloys and their mechanical properties, forming processes and metal cutting shaping processes are very complex.

U.S. Pat. No. 6,043,451 A discloses a process for plasma coating of components and for spray compacting of nickel-titanium alloy foils. The metal is supplied to a plasma torch as a powder or wire in the process known from U.S. Pat. No. 6,043,451 A. The production of powder and wire is very complex and expensive and requires at least one production stage proceeding from a (large-format) semifinished product. For powdered metal there is moreover the increased danger of absorption of oxygen due to the large surface.

The disadvantage in the process known from U.S. Pat. No. 6,043,451 A is the formation of a conical spray jet of molten metal caused by the radial symmetry of the plasma torch, by which wider foils or coatings can only be produced by overlapping of several spray cones or repeated spraying with the same spray cone. The layers produced in this way have an undesirable, nonuniform surface profile (compare FIG. 2 a). The production output at 3 kg/h (50 g/min) is very low and is thus poorly suited to producing thicker foils or coatings or semifinished products such as pins, pipes, or sheets.

Atomization of liquids by gas atomization is known.

For example, DE 197 58 111 A discloses a process for producing metal powders. In these known processes the metal melt emerges in the form of a film from a nozzle with a slotted exit opening. The film is stabilized by a laminar gas flow in a Laval gas nozzle and is then finely atomized. The productivity of the nozzle system can be changed at will by lengthening the nozzle slot without adverse effects on the powder quality. When melted in tanks however there is fundamentally the danger of contamination of the metal powder which has been obtained due to the materials of the tank.

DE 41 02 101 A discloses a process in which metals in the form of a vertically arranged bar with radial-symmetrical cross section are melted off under an inert atmosphere by induction on the bottom end. The melt drips under the influence of gravity and electromagnetic pressure (resulting from the induction coil). The drops are then atomized by a gas flow emerging from an annular gap nozzle into a relatively coarse powder with an average grain size of roughly 50 microns with a wide grain size distribution. The metal bar is turned around its longitudinal axis during melting-off and guided into the induction coil according to consumption. For this purpose a complex drive is necessary. Gas consumption per kilogram of metal powder is high. Fine powders with a grain size less than 30 microns can only be produced with low yield. The total productivity of the process known from DE 41 02 101 A is low at roughly 20 kg/hour and cannot be increased without adverse effects on powder quality.

The object of this invention is to make available a process of the initially mentioned type with which direct conversion of metal which is present for example as a commercially available semifinished product into powders, metal foils, surface coatings or products of another format (semifinished products) is possible with high productivity, good economy and without the danger of introducing impurities.

This object is achieved with a process which has the features of claim 1.

Advantageous and preferred embodiments of the process as claimed in the invention are the subject matter of the dependent claims.

In the process as claimed in the invention, metal in the form of a commercially available semifinished product which has for example the shape of a cuboid is melted without contact and atomized into a linear, especially wedge-shaped spray jet. This spray jet is used to produce the desired metal product. With the process as claimed in the invention various products can be produced from metal. For example, using the process as claimed in the invention metal powders can be produced, its being especially advantageous in embodiments that producing metal powders from reactive metals is possible by the process as claimed in the invention. In any case it is ensured that in the process as claimed in the invention metal impurities are precluded or are for the most part prevented. With the process as claimed in the invention also other metal products can be produced by for example surfaces being coated or semifinished products such as foils, sheets or pins being produced.

For example, in the process as claimed in the invention the metal of the semifinished product can be melted, atomized, sprayed onto a carrier and hardened on the carrier. The process as claimed in the invention can also be used for example for coating of workpieces.

In an embodiment of the process as claimed in the invention which is designed for producing metal powders, a semifinished product, for example a pin, of metal which has an essentially rectangular cross sectional shape can be inductively melted on the surface of the two lengthwise sides of its front. The front side which is melting off is located within a laminar gas flow of a linear nozzle. The two halves of the linear Venturi nozzle consist preferably of a material which does not couple to the magnetic field of the induction heating.

In one embodiment of the invention, tubes of metal, preferably copper, are embedded in the Venturi half nozzle and are used as conductors for the inductive exciter current with simultaneous cooling by a cooling fluid, for example, water. The tubes are for example each connected to one another via other tubes on the ends of the Venturi half nozzle.

In this embodiment, the gas flows extend over the melting surface of the semifinished product which is supplied in the form of a pin and convey the melt in the form of two very thin films to the tip of the pin. The two films combine here and the resulting melt film is further stabilized by the laminar gas flow, accelerated and finally atomized into fine droplets.

In the invention the liquid (melt) film need not emerge from the nozzle with motion directed down. The process as claimed in the invention works independently of the location, therefore not only vertically up, but also horizontally or vertically down, and in any other alignment.

The guidance of the liquid film, especially of the film of metal melt, by the gas flow is stronger than the force of gravity acting on the melt. The independence of the location of the atomizing nozzle gives to the designer of nozzle systems as claimed in the invention creative degrees of freedom which can be used in a reduction of the overall height of the system.

In one embodiment, the process as claimed in the invention is carried out in a tank, in the embodiment essentially continuous production of metal products being possible by a new semifinished product being connected to the semifinished product which has almost been consumed by melting off, for example connected by a weld. By repeatedly adding and welding on other semifinished products, especially a semifinished product in the form of metal bars, the actual atomization process can be carried out continuously and economically.

Other details and features of the process as claimed in the invention will become apparent from the following description with reference to the drawings.

FIG. 1 schematically shows one arrangement for executing the process as claimed in the invention,

FIG. 2 shows another arrangement for executing the process as claimed in the invention,

FIG. 3 a shows a coating as is available in the prior art (U.S. Pat. No. 6,043,451 A) and

FIG. 3 b shows a coating as can be produced in an application of the process as claimed in the invention.

The arrangement shown in FIG. 1 is a sample application of the process as claimed in the invention for producing a foil from metal. This arrangement consists of a longish (linear) gas nozzle 1 in which there are water-cooled copper tubes 2. The copper tubes 2 are used to produce an inductive magnetic field. The semifinished product 3 of metal to be processed with an essentially rectangular cross section is inserted into the elongated input opening of the gas nozzle 1 and is melted under the action of the inductive magnetic field without contact on its lengthwise sides.

A gas flow 4 which is directed by a means which is not detailed at the elongated mouth of the gas nozzle 1 and which is preferably symmetrical, therefore pointed from the two sides of the semifinished product 3 into the gas nozzle 1, entrains the molten metal and conveys it with formation of a thin film 5 through the mouth the gas nozzle 1. The gas nozzle 1 used in the invention can be made as a Laval nozzle or as a Venturi nozzle. After passage through the narrowest point of the gas nozzle 1 (its elongated mouth) the film 5 of metal melt is atomized into a linear, wedge-shaped, especially tent-shaped spray jet 6. The spray jet 6 in this embodiment is pointed at a continuous and cooled metal belt 7 as the carrier.

The droplets of molten metal are liquid or still at least partially liquid at the time of impact on the metal belt 7 and solidify into a metal foil 8 with a homogenous surface (except for the two edges). The metal foil 8 can be wound into a roll 9 of foil after its complete solidification which can be supported by forced cooling, and detachment from the metal belt 7.

By matching the length of the spray jet 6 to the entire width of the surface of the carrier 7, for example of the endless metal belt 7 or of the semifinished product—except for the two edges—metal can be applied to the carrier 7 in a uniform thickness.

FIG. 3 a shows the spray result with a conventional round nozzle (compare U.S. Pat. No. 6,043,451 A) in which several metal beads 1 to 4 are sprayed next to one another. FIG. 3 b shows a metal foil 8 which has been produced with the process as claimed in the invention, in which in a single spraying process a uniformly thicker metal layer (foil 8) is formed.

The productivity of the process of the invention can be optionally set via the length of the spray jet and via the melting heat output of the induction heating.

The metal added as raw material preferably in the form of a semifinished product is converted into the desired end product in one working cycle, therefore comes into contact only with the atomization gas and when the purity of the gas atmosphere is high enough, can be converted into the metal product without an increase of impurities.

In the process as claimed in the invention, in one embodiment reactive metal or alloy is thermally compacted by spray compacting, the parent material in the form of the semifinished product being melted without contact, especially inductively, and atomized into a linear, wedge-shaped spray jet. The particles of the spray jet are allowed to solidify for example into a metal powder, are spray compacted on a substrate for a product, or are applied as a surface coating to a component.

With the process as claimed in the invention, any metals, especially reactive metals such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhenium or an alloy based on these metals can be processed.

In particular, the process as claimed in the invention is suited for processing of a nickel-titanium alloy or a superalloy based on nickel or cobalt.

In one embodiment of the process as claimed in the invention the semifinished product to be processed is a composite material of high-melting phases and a low-melting binder matrix. The high-melting phase can be a carbide.

With the process as claimed in the invention among others products in the form of foils, sheets, tubes or pins can be produced.

One advantage of the process as claimed in the invention is that the purity of the product differs only slightly from the purity of the parent material (semifinished product).

In the process as claimed in the invention it is possible to continuously control the productivity per unit of length of the spray jet via the supplied heating output (inductive heating).

In the process as claimed in the invention it is also possible to spray several elongated spray jets in succession onto the same substrate to achieve higher layer thickness.

In one embodiment of the process as claimed in the invention, in addition to the spray jet of molten metal in the form of droplets dispersoids are added specifically via another nozzle. These dispersoids can be the following for example: silicon carbide, tungsten carbide, corundum (Al₂O₃) or zirconium oxide. The purpose of adding these dispersoids and other additives which can also be volatile is to influence the properties of the process product in the desired direction.

In order to simplify detachment of the product from the carrier 7 (substrate), a separating agent can be applied to the substrate before spray compacting.

The process as claimed in the invention can be carried out especially as described below on one example in the production of a metal powder.

In a gas-tight, argon-filled tank which is at the same pressure as the tank vicinity, a titanium bar with a rectangular cross section (initial dimensions: width 50 mm, thickness 40 mm, length 3000 mm) is floating zone-melted with an induction frequency of 350 kHz and atomized at 5 kg/min. When the first bar has been melted off to a length of 500 mm, a new bar, after it has passed the preliminary lock chamber with being rendered inert and pressure equalization is brought to the end of the first bar facing away from the atomization, and the two bars are welded to one another linearly by means of a laser jet without filler on their two sides facing away from the melting assembly. The weld keeps the two bars together until it finally reaches the melting zone itself and is melted at the same time. By repeatedly adding and welding on a new metal bar the actual atomization process can be carried out continuously and economically. At a gas pressure of 30 bar in the pipeline in front of the linear gas nozzle a powder with an average grain size of 9.0 microns is obtained.

A device which is suitable for this purpose for example is shown in FIG. 2. This device has a linear gas nozzle 10 with internal supply of the primary atomization gas 13. An induction coil 12 is integrated into the linear gas nozzle 10. As shown schematically in FIG. 2, primary atomization gas 13 emerges from the linear gas nozzle 10, symmetrically in the illustrated embodiment, so that there are two streams of primary atomization gas 13.

Furthermore, in the linear gas nozzle 10 there is a secondary gas flow 14 which forms a melt film 21 on the metal which melts off the metal bar 15 with a rectangular cross section. In this embodiment the melting metal bar 15 is advanced by rotationally driven guide rolls 18 toward the gas nozzle 10.

The primary gas flows 13 are produced by the atomization gas which is supplied primarily within the gas nozzle 10. The primary gas flows 13 produce a local underpressure by which gas is intaken which forms the secondary gas flows 14 which are used as the support gas.

The entire arrangement is accommodated in a housing 19 which is filled with an inert gas, especially argon, the gas in the housing 19 being at the same pressure as the tank vicinity.

The metal bar 15 can be for example a titanium bar. Under the action of the primary atomization gas flows 13 a spray jet of metal droplets 22 is formed from the melt film 21. These droplets of molten metal 22 can solidify into a powder, or, as is described by FIGS. 1 and 2 b, can be spray-compacted.

In order to enable more or less continuous operation, in the process as claimed in the invention, as indicated in FIG. 2, another metal bar with a rectangular cross section can be added onto the melting metal bar 15 by the former bar being connected to the metal bar 15 by two welds 17 which are aligned especially parallel to the plane of the drawing in FIG. 2. The following metal bar 16 is likewise guided by rotationally driven guide rolls 18. Following the linear gas nozzle 10 there is another tank 25 in which the molten metal divided into droplets (metal droplets or powder products 22) hardens into a metal powder.

In summary, one embodiment of the invention can be described as follows:

To produce products of metal, such as powders, foils, coatings and molded parts, such as pins, pipes or sheets, from metal in the form of a semi-finished product 15, the metal of the semifinished product 15 is melted by an inductive magnetic field 12, atomized and allowed to solidify in a chamber 25 into a powder or is sprayed onto a carrier and hardened on the carrier. The molten metal is supplied in a gas nozzle 10 which is made either as a Laval nozzle or as a Venturi nozzle, as a film 21 which is stabilized by gas flows 14, and is then atomized by other gas flows 13. 

1. Process for producing products from metal, especially powders, foils, coatings and molded parts, such as pins, pipes or sheets, from metal in the form of a semifinished product, in which the metal of the semifinished product is melted and atomized and finally hardened again, characterized in that the metal is melted without contact and is atomized by gas flows acting on the melt.
 2. Process as claimed in claim 1, wherein the metal is inductively melted.
 3. Process as claimed in claim 1, wherein the metal is supplied in the form of blocks which are melted off in the area of their sides.
 4. Process as claimed in claim 1, wherein the molten metal is atomized in a gas nozzle into a spray jet by at least one gas flow being supplied to the gas nozzle.
 5. Process as claimed in claim 1, wherein two gas flows from opposing sides are supplied to the gas nozzle.
 6. Process as claimed in claim 1, wherein the molten metal is atomized into an elongated spray jet.
 7. Process as claimed in claim 1, wherein the molten metal of the spray jet is allowed to harden into a powder.
 8. Process as claimed in claim 1, wherein the molten metal in the form of a spray jet is hardened on a carrier.
 9. Process as claimed in claim 8, wherein the hardened metal is removed from the carrier as a metal foil.
 10. Process as claimed in claim 8, wherein the hardened metal on the carrier forms a coating which remains on the latter.
 11. Process as claimed in claim 8, wherein spray compacting of metal is repeated to attain higher layer thicknesses of the product.
 12. Process as claimed in claim 1, wherein the melted metal is atomized into an elongated spray jet over a width which is at least as great as the width of the product to be produced.
 13. Process as claimed in claim 12, wherein the melted metal is atomized in a longish gas nozzle into a spray jet.
 14. Process as claimed in claim 1, wherein processing is done for a nickel-titanium alloy.
 15. Process as claimed in claim 1, wherein at least one metal from the group consisting of iron, copper, aluminum, zinc, tin, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhenium or an alloy based on at least two of these metals is processed.
 16. Process as claimed in claim 1, wherein the metal being processed is a superalloy based on nickel or cobalt.
 17. Process as claimed in claim 1, wherein the metal being processed is a composite material of a high melting phase and a low melting binder matrix.
 18. Process as claimed in claim 17, wherein the high melting phase is a carbide, oxide or nitride.
 19. Process as claimed in claim 1, wherein dispersoids are added from another nozzle in addition to the spray jet of molten metal in the form of droplets.
 20. Process as claimed in claim 19, wherein carbides, oxides and/or nitrides are added as the dispersoids.
 21. Process as claimed in claim 1, wherein a separation agent is applied to the carrier in the production of a semifinished product before spray compacting.
 22. Process as claimed in claim 1, wherein the semifinished product of metal is supplied in the form of bars, especially cuboidal bars.
 23. Process as claimed in claim 1, wherein the semifinished product before it is used up is joined to another semifinished product which is connected to the almost consumed semifinished product.
 24. Process as claimed in claim 23, wherein the semifinished products are joined to one another by welding.
 25. Process as claimed in claim 1, wherein the metal is supplied in a protective gas-filled housing to the gas nozzle.
 26. Process as claimed in claim 1, wherein following the gas nozzle there is a tank in which the melted metal is cooled into a powder.
 27. Process as claimed in claim 1, wherein the molten metal is formed into a film of metal melt by secondary gas flows in the region upstream from and in the gas nozzle.
 28. Process as claimed in claim 27, wherein the film of metal melt is formed and stabilized by two secondary gas flows which are symmetrical with respect to the gas nozzle.
 29. Process as claimed in claim 27, wherein the film of metal melt is atomized by primary gas flows after passage through the gas nozzle. 